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| United States Patent |
5,646,331
|
|
Thiel
, et al.
|
July 8, 1997
|
Process for working up residues from the raw ester distillation in the
preparation of dimethyl terephthalate(DMT)
Abstract
A process for working up, by methanolysis, a residue fraction resulting
from raw ester distillation in a DMT process is provided, the process
involving:
admixing the residue fraction with liquid or gaseous methanol, or both to
give a reaction mixture;
providing the reaction mixture to a reactor, wherein, prior to entry into
the reactor, the reaction mixture is at a temperature of from 230.degree.
to 265.degree. C.; and
methanolysing the reaction mixture in the reactor, wherein a bottom portion
of the reactor is maintained at a temperature of from 230.degree. to
265.degree. C.
| Inventors:
|
Thiel; Ralf (Niederkassel, DE);
Auschner; Reinhard (Troisdorf, DE)
|
| Assignee:
|
Huels Aktiengesellschaft (Marl, DE)
|
| Appl. No.:
|
695777 |
| Filed:
|
August 12, 1996 |
Foreign Application Priority Data
| Aug 23, 1995[DE] | 195 30 970.7 |
| Current U.S. Class: |
560/78; 560/98; 560/99 |
| Intern'l Class: |
C07C 069/82 |
| Field of Search: |
560/78,98,99
|
References Cited
U.S. Patent Documents
| 4126755 | Nov., 1978 | Bunger et al. | 560/77.
|
| 5286896 | Feb., 1994 | Korte et al. | 560/77.
|
| 5338882 | Aug., 1994 | Korte et al. | 562/483.
|
| Foreign Patent Documents |
| 0 053 241 | Jun., 1982 | EP.
| |
| 0 464 046 | Jan., 1992 | EP.
| |
| 11 42 858 | Jan., 1963 | DE.
| |
| 2 010 137 | Sep., 1971 | DE.
| |
| 22 44 662 | Apr., 1974 | DE.
| |
| 40 26 733 | Feb., 1992 | DE.
| |
| WO90/09367 | Aug., 1990 | WO.
| |
Primary Examiner: Barts; Samuel
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.
Claims
What is claimed as new and desired to be secured by Letters Patent of the
United States is:
1. A process for working up a residue fraction resulting from raw ester
distillation in a DMT process, comprising:
admixing the residue fraction with liquid or gaseous methanol, or both to
give a reaction mixture;
providing the reaction mixture to a reactor, wherein, prior to entry into
the reactor, the reaction mixture is at a temperature of from 230.degree.
to 265.degree. C.; and
methanolysing the reaction mixture in the reactor, wherein a bottom portion
of the reactor is maintained at a temperature of from 230.degree. to
265.degree. C.
2. The process according to claim 1, wherein, prior to entry into the
reactor, the reaction mixture is at a temperature of from 245.degree. to
255.degree. C.
3. The process according to claim 1, wherein the bottom portion of the
reactor is maintained at a temperature of from 245.degree. to 255.degree.
C.
4. The process according to claim 1, wherein the methanolysing step is
carried out at a pressure of from 1 to 40 bar abs.
5. The process according to claim 4, wherein the methanolysing step is
carried out at a pressure of from 1 to 3 bar abs.
6. The process according to claim 1, wherein the reaction mixture of the
residue fraction admixed with methanol is brought to the required
temperature using a heat exchanger prior to entry into the reactor.
7. The process according to claim 6, wherein the heat exchanger is operated
by a means selected from the group consisting of electricity,
high-pressure steam, preheated heat-transfer oil and combinations thereof.
8. The process according to claim 1, wherein the bottom portion of the
reactor is heated by a means selected from the group consisting of
electricity, high-pressure steam, preheated heat-transfer oil and
combinations thereof.
9. The process according to claim 1, wherein during the methanolysing step,
additional methanol is added to the reactor.
10. The process according to claim 9, wherein the additional methanol is
added as liquid methanol.
11. The process according to claim 9, wherein the additional methanol is
added as gaseous methanol.
12. The process according to claim 1, wherein the reactor is a reaction
distillation column.
13. The process according to claim 1, wherein the methanolysing step
results in production of reactor bottoms, which are not subjected to
circulation.
14. The process according to claim 1, wherein the methanolysing step
results in production of reactor bottoms, wherein a portion of the reactor
bottoms is bled off and subjected to a catalyst recovery step.
15. The process according to claim 1, wherein said methanolysing step is a
single stage methanolysis.
16. The process according to claim 1, wherein products from the
methanolysing step are recirculated to the DMT process.
17. The process of claim 16, wherein the products from the methanolysing
step are passed through a dephlegmator prior to recirculation to the DMT
process.
18. The process of claim 1, wherein, prior to entry into the reactor, the
reaction mixture is at a temperature of from 245.degree. to 255.degree.
C., wherein the bottom portion of the reactor is maintained at a
temperature of from 245.degree. to 255.degree. C., and wherein the
methanolysing step is carried out at a pressure of from 1 to 40 bar abs.
19. The process of claim 18, wherein the methanolysing step is carried out
at a pressure of from 1 to 40 bar abs.
20. The process of claim 1, wherein fill level of the reaction mixture in
the reactor is maintained at an essentially constant level.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for working up a residue
fraction by methanolysis, the residue fraction originating from the raw
ester distillation of the DMT production process.
2. Discussion of the Background
Dimethyl terephthalate (DMT) is produced on a large scale in numerous
plants around the world. DMT is an important starting compound for the
preparation of polyesters. Polyesters have long been used in the
preparation of fibers, films, including photographic films, magnetic tapes
or bottles of polyethylene terephthalate, to name only a few.
The conventional Witten DMT process essentially comprises (cf. EP-B 0 464
046, DE-A 40 26 733) the process steps of
(1) Oxidation of para-xylene (p-X) and methyl para-toluate (p-TE) with
downstream waste gas purification;
(2) Esterification of the reaction products from the oxidation with
methanol;
(3) Separation of the raw ester formed into
a) a fraction which is recirculated to the oxidation,
b) a raw DMT fraction containing more than 99% by weight of DMT and
c) a high-boiling residue fraction including its work-up; and
(4) Purification of the raw DMT fraction, for example by washing,
recrystallization or pure distillation.
It is also possible to prepare terephthalic acid from DMT-rich fractions by
targeted hydrolysis.
The oxidation of a mixture of para-xylene (p-X) and methyl para-toluate
(p-TE or pT ester) is generally carried out in the liquid phase using
atmospheric oxygen in the presence of a heavy metal catalyst (DE-C 20 10
137) at a temperature of from about 140.degree. to 180.degree. C. and a
pressure of from about 4 to 8 bar abs. The oxidation step results in a
reaction mixture which contains predominantly monomethyl terephthalate
(MMT), p-toluic acid (p-TA) and terephthalic acid (TA) dissolved or
suspended in p-TE. This mixture is esterified with methanol at a
temperature of from about 250.degree. to 280.degree. C. at a pressure of
from 20 to 25 bar abs. The raw ester obtained is separated by distillation
into a p-TE fraction, a raw DMT fraction and a high-boiling,
catalytic-containing residue fraction. The p-TE fraction is recirculated
to the oxidation and the raw DMT fraction is converted via subsequent
purification steps into the desired product quality.
The residue fraction originating from the raw distillation is generally
further treated by methanolysis. FIG. 1 shows the flow diagram of a
conventional single-stage methanolysis. In the reaction distillation
column (1.1), the residues (1.2) and superheated methanol vapour (1.3) are
continuously introduced in countercurrent at atmospheric pressure. The
bottom of the column is additionally heated by means of a heat-transfer
oil (1.4). The methanolysis is carried out at a temperature of from
265.degree. to 280.degree. C. Part of the residues are converted into
materials which can be reused in the process. Acids present in the residue
fraction are esterified in the methanolysis, part of the high-boiling
organic compounds are dissociated and organic products of value already
present are separated from undesired organic compounds which can no longer
be used. The valuable or useful materials thus obtained go together with
excess methanol via the top of the column to the dephlegmator (1.5) and
are subsequently recirculated to the process, i.e. to the oxidation (1.6).
The bottom residue formed in the methanolysis is generally conveyed to
catalyst recovery (1.7). It is found in practice that carbon deposits and
blockages occur in the reaction column during methanolysis, resulting in
more frequent and unplanned stoppages of the unit.
EP-B 0 464 046 discloses a two-stage methanolysis for working up the
residue from the raw ester distillation. The first methanolysis stage
essentially comprises a reactor with upstream heat exchanger and
circulation system, with a distillation column arranged downstream of the
top of the reactor. In contrast to the first stage, the second
methanolysis stage comprises a reaction distillation column. In both
stages, additional methanol in vapour form is fed into the respective
residue circuit upstream of the heat exchangers, i.e. before entry into
the respective methanolysis reactors. The operating temperatures for both
methanolysis stages are more than 265.degree. C.; particularly in the
second methanolysis stage. Furthermore, each methanolysis stage is
operated as a circuit and only partial streams are replaced, with the
throughput amounts of the residue fraction to be worked up naturally being
comparatively small. In addition, the two-stage methanolysis requires a
high investment and high maintenance cost for operation of the plant.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a process
which makes it possible to work up the residue fraction formed in the raw
ester distillation during DMT production in a very economical manner.
A further object of the present invention is to provide a process for
working up the residue fraction from raw ester distillation in the DMT
process by methanolysis which reduces carbon deposits in the reactors and
pipe systems of the methanolysis.
These and other objects of the present invention have been satisfied by the
discovery that carbon deposits can be substantially reduced in a
methanolysis procedure wherein liquid and/or gaseous methanol are added to
the raw ester distillate, the temperature of the mixture before entry into
the methanolysis reactor is maintained at from 230.degree. to 265.degree.
C. and the temperature in the bottom of the reactor is controlled to be
from 230.degree. to 265.degree. C., and wherein, in the present process,
the reactor bottoms are preferably not subjected to a throughput-reducing
circulation procedure, by means of which the capacity of the methanolysis
stage is able to be significantly increased.
BRIEF DESCRIPTION OF THE FIGURES
A more complete appreciation of the invention and many of the attendant
advantages thereof will be readily obtained as the same becomes better
understood by reference to the following detailed description when
considered in connection with the accompanying Figures, wherein:
FIG. 1 shows a flow diagram of a conventional single-stage methanolysis
procedure.
FIG. 2 shows a flow diagram of a preferred embodiment of the methanolysis
procedure of the present invention.
DETAILED DISCUSSION OF THE PREFERRED EMBODIMENTS
The present invention relates to a process for working up a residue
fraction resulting from raw ester distillation in a DMT process,
comprising:
admixing the residue fraction with liquid or gaseous methanol, or both to
give a reaction mixture;
providing the reaction mixture to a reactor, wherein, prior to entry into
the reactor, the reaction mixture is at a temperature of from 230.degree.
to 265.degree. C.; and
methanolysing the reaction mixture in the reactor, wherein a bottom portion
of the reactor is maintained at a temperature of from 230.degree. to
265.degree. C.
In the present process liquid and/or gaseous methanol is preferably fed
into the residue fraction. Before entry into the methanolysis reactor, the
mixture has a temperature of from 230.degree. to 265.degree. C.,
preferably from 245.degree. to 255.degree. C. The temperature in the
bottom of the reactor is maintained at from 230.degree. to 265.degree. C.,
preferably from 245.degree. to 255.degree. C. The methanolysis is
preferably carried out at a pressure of from 1 to 40 bar abs., more
preferably at a pressure of from 1 to 3 bar abs.
FIG. 2 shows a flow diagram of a preferred embodiment of the process of the
invention, where the methanolysis is carried out, in particular, in a
single stage and the residue fraction from the raw ester distillation
(2.1.1) is preferably worked up in a reaction distillation column (2.2).
Preferably, the residue fraction admixed with methanol (2.1.2) is passed
through a heat exchanger (2.3) before entry into the methanolysis reactor,
in order to provide the mixture of residue fraction and methanol (2.1.2)
to the reaction distillation column (2.2) at the required temperature. The
heat exchanger can be any conventional heat exchanger and is preferably
operated electrically, by means of high-pressure steam, by means of a
preheated heat-transfer oil, for example MARLOTHERMR, or a combination
thereof. In the process of the invention, the bottom of the reactor (2.4)
can also be heated using any conventional heating means. The bottom of the
reactor (2.4) is preferably heated electrically, by means of high-pressure
steam, by means of a preheated heat-transfer oil or a combination thereof.
As described above, the residue fraction pretreated with methanol is
preferably reacted in the methanolysis reactor with further addition of
methanol (2.5). Here too, the methanol can be fed in liquid and/or vapour
form.
In the process of the present invention, the reaction is carried out under
particularly gentle and comparatively mild conditions relative to
conventional methanolysis procedures. The useful products obtainable in
the reaction are preferably recirculated via a dephlegmator (2.6) to the
DMT process, i.e. to the oxidation stage (2.7). In the process of the
present invention, the proportion of useful product in the bottom of the
reaction column can be reduced to about 3.5% compared with about 8.5% in
conventional processes. In the present process, part of the reactor
bottoms can be bled off and fed to a process for catalyst recovery (2.8)
if desired. The fill level in the bottom of the reactor should preferably
be kept essentially constant. The fill level can, for example, also be
regulated by means of the amount of feed of the residue from the raw ester
distillation. Portions can be taken from the bottom of the reactor
continuously or discontinuously. Catalyst recovery is carried out, for
example, by extraction (cf. EP-B 0 053 241).
By means of the gentle procedure in the process of the present invention,
it is possible to increase the reaction time of the methanolysis reaction,
which represents a further economic advantage. Particularly surprising is
the fact that the process of the present invention enables the yield over
the entire DMT process to be increased by about 0.5%, even without
circulation of the reactor bottoms. This is a significant commercial
improvement in the economics of the process.
Legends for FIG. 1
Flow diagram of a conventional single-stage methanolysis reactor for
working up a residue from the raw ester distillation in the preparation of
DMT
1.1 Reaction distillation column
1.2 Feed of the residue from the raw ester distillation
1.3 Gaseous methanol feed
1.4 Heating of the bottom of the reaction column
1.5 Dephlegmator
1.6 Recirculation of the useful product to the process
1.7 Substream of the bottom residue to catalyst recovery
Legends for FIG. 2
Flow diagram of a single stage methanolysis reactor of the present
invention for working up a residue from the raw ester distillation in the
preparation of DMT
2.1 Feed of the residue (2.1.1) from the raw ester distillation, to which
methanol (2.1.2) has been added
2.2 Reaction distillation column
2.3 Heat exchanger
2.4 Heating of the bottom of the reaction column
2.5 Methanol feed
2.6 Dephlegmator
2.7 Recirculation of the useful products to the process
2.8 Substream of the bottom residue to catalyst recovery
This application is based on German Patent Application 195 30 970.7, filed
with the German Patent Office on Aug. 23, 1995, the entire contents of
which are hereby incorporated by reference.
Obviously, additional modifications and variations of the present invention
are possible in light of the above teachings. It is therefore to be
understood that within the scope of the appended claims, the invention may
be practiced otherwise than as specifically described herein.
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